Fix change_connection by conn -> old_conn

Not sure this code path is ever hit currently since we have factored
everything to add/delete, but keeping in just in case

vistrails/core/vistrail/pipeline.py

@@ -446,7 +446,7 @@ def change_connection(self, old_id, c, *args):
                                    old_conn.id)
             if self.graph.out_degree(old_conn.sourceId) < 1:
                 self.modules[old_conn.sourceId].connected_output_ports.discard(
-                    conn.source.name)  # FIXME : conn is c? old_conn?
+                    old_conn.source.name)
             if self.graph.in_degree(old_conn.destinationId) < 1:
                 connected_input_ports = \
                     self.modules[old_conn.destinationId].connected_input_ports